Flywheel and molten salt hybrid energy storage systems

ABSTRACT

This disclosure describes novel hybrid energy storage systems for providing short-term and long-term storage and delivery of electricity generated by any energy source including renewable energy sources such as solar energy and wind energy. The hybrid energy storage systems described herein have a higher overall real-world efficiency than energy storage systems currently available.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.17/530,219 filed on Nov. 18, 2021, which claims the benefit of U.S.Provisional Application Ser. No. 63/118,393, filed Nov. 25, 2020. Thedisclosures of the prior applications are considered part of (and areincorporated by reference in) the disclosure of this application.

BACKGROUND 1. Technical Field

This disclosure relates to novel energy storage systems for long-termstorage of electricity generated by any energy source includingrenewable energy sources such as solar energy and wind energy.

2. Background Information

Rapid growth of renewable energy sources such as solar energy and windenergy is a powerful driving force for cost effective and long-termstorage of large amount of renewable energy. There are varieties ofdifferent energy storage systems (ESS) readily available in the markets.Major forms of ESS in the markets are Battery Energy Storage Systems,Flywheel Energy Storage Systems (FESS), Molten Salt Energy StorageSystems (MSESS), and Compressed Air Energy Storage (CAES). Each of theseenergy storage systems have characteristic advantages and disadvantages.However, as this disclosure shows, a novel integration of such storagesystems solves problems that are not solved by such stand-alone systems.

SUMMARY

This disclosure describes novel ways of integrating FESS and MSESS intoan integrated flywheel and molten-salt hybrid energy storage system(hereinafter referred to as “YKESS”). The YKESS synergistically enhancesthe advantages of FESS and MSESS technologies, as described furtherbelow. YKESS is a novel integration of FESS and MSESS that enhances thestrengths of FESS and MSESS and eliminates or minimizes the weaknessesof FESS and MSESS. For convenience, end users of electricity and networkgrids of electrical power will be referred to hereinafter as “users.”

In one aspect, this disclosure is directed to an energy storage anddelivery system that includes one or more FESS and a MSESS. The one ormore FESS are configured to generate and deliver electricity to users.The MSESS includes a container holding salt therein; a heater in thermalcontact with the salt; a heat exchanger in thermal contact with thesalt; a turbine fluidly coupled to an output of the heat exchanger; anelectricity generator mechanically coupled to the turbine, whereinelectricity output by the electricity generator is delivered to powerflywheel rotations of the one or more FESS; and a compressor or pumpfluidly coupled to an input of the heat exchanger, wherein thecompressor or pump is powered by energy output by the one or more FESS.

Such a system may optionally include one or more of the followingfeatures. The heater may be configured to be electrically powered byelectricity generated by solar panels or wind turbines. The system mayalso include the solar panels or the wind turbines. The system may beconfigured to send at least a first portion of the electricity generatedby the solar panels or the wind turbines to users without powering theone or more FESS. The system may be configured to send at least a secondportion the electricity generated by the solar panels or the windturbines to the one or more FESS to power flywheel rotations of the oneor more FESS. The heater may be configured to transfer heat to the salt.The heat exchanger may be configured to transfer heat from the salt to afluid passing through the heat exchanger as the fluid flows toward theturbine. The system may also include a fluid storage tank positionedfluidly between an output of the turbine and an input of the compressoror pump. In some embodiments, the fluid storage tank is underground. Insome embodiments, excess electricity from a renewable energy source isdistributed to the one or more FESS and the MSESS system such thatenergy stored in the MSESS is at least two times greater than energystored in the one or more FESS. The MSESS may also be configured todeliver energy to power the compressor or pump. The system may beconfigured to automatically switch the energy to power the compressor orpump from being delivered by the one or more FESS to being delivered bythe MSESS in response to the energy contained by the one or more FESSreaching a lower limit. The electricity output by the electricitygenerator may also be delivered to the users. In some embodiments, eachFESS of the one or more FESS stores 10 kWh or less of kinetic energy.Each FESS of the one or more FESS may weigh less than 1,000 kg. The oneor more FESS comprises at least four FESS.

In another aspect, this disclosure is directed to an energy storagesystem for use during interruptions of electricity from a renewableenergy source. The system includes a FESS for short-time delivery ofelectricity to users and an MSESS for long-time delivery of electricityto the users.

Such a system may optionally include one or more of the followingfeatures. The MSESS may include an air compressor, and the FESS may beconfigured to deliver energy to power the air compressor. In someembodiments, the MSESS is also configured to deliver energy to power theair compressor. The energy storage system may be configured toautomatically switch the energy to power the air compressor from beingdelivered by the FESS to being delivered by the MSESS in response to theenergy contained by the FES system reaching a lower limit.

In another aspect, this disclosure is directed to a method of deliveringelectricity to users. The method includes: (i) delivering a firstportion of electricity generated from a renewable energy source to theusers; (ii) delivering a second portion of electricity generated fromthe renewable energy source to one or more FESS; and (iii) delivering athird portion of electricity generated from the renewable energy sourceto a MSESS.

Such a method may optionally include one or more of the followingfeatures. The additional electricity may be delivered to the one or moreFESS in response to a kinetic energy level of the one or more FESS beingbelow a pre-determined lower threshold value. The method may alsoinclude powering, by energy delivered from the one or more FESS, a pumpor compressor of the MSESS. The method may also include, in response toa demand for the electricity from the users being greater than the firstportion of electricity generated from the renewable energy source,delivering additional electricity to the users, wherein the additionalelectricity is generated by the one or more FESS. The method may alsoinclude, in response to a demand for the electricity from the usersbeing greater than the first portion of electricity generated from therenewable energy source, delivering additional electricity to the users,wherein the additional electricity is generated by the MSESS.

There are at least five major objectives and advantages that the YKESSdescribed herein provides. The first objective and advantage islong-term and low-cost storage of renewable energy harvested from solarpanels, wind turbines, and/or other forms of renewable energy devices.In this context, “long-term” means many days or many weeks of time.“Low-cost” means an initial capital cost of storage per kilowatt-hourthat is significantly lower than cost of storage per kilowatt-hour bymodern, commercially available FESS.

The second objective and advantage is that the YKESS described hereincan use one or many low-cost, maintenance-free, and small-sized FESSe.g., with an energy storage capacity of about 10 kWh (kilowatt-hour).

Third, the YKESS described herein can provide essentially instantdelivery of sufficient electric power to energy users to meet the widelyand frequently fluctuating demand of electricity from the users.

Fourth, the YKESS described herein has the capability of sendingsufficient electric power to one or many users continuously for one weekor longer.

Fifth, the YKESS described herein can provide large-scale energy storageof renewable energy without using environmentally and biologicallyharmful materials. In addition, the YKESS only occupies a small area ofland.

The YKESS described herein can be an indispensable source of energy forlarge data centers, large manufacturing plants, big hospitals, and thelike. Moreover, the YKESS can provide a backup of the power grid (e.g.,for large cities) in case an existing supply of electricity is disruptedby a brownout, or for a long period.

The YKESS described herein can also reduce the heavy dependence ofpower-hungry societies on energy generated by burning fossil fuels.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this disclosure pertains. In addition, the materials,methods, and examples of the embodiments described herein areillustrative only and not intended to be limiting.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description herein. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating an example YKESS thatintegrates FESS and MSESS in accordance with some embodiments describedherein.

FIG. 2 is a schematic diagram illustrating another example YKESS thatintegrates FESS and MSESS in accordance with some embodiments describedherein.

FIG. 3 is a schematic diagram of another example YKESS in which manysmall low-cost units of FESS are connected to a single MSESS having alarge energy storage capacity.

DETAILED DESCRIPTION

This disclosure describes new and novel systems that integrate FESS andMSESS into a single energy storage system that is referred to herein asthe YKESS. The integrated/hybrid energy storage systems described herein(YKESS) enhance the individual advantages of FESS and MSESS and avoidthe individual disadvantages of FESS and MSESS. In some cases, the YKESSdescribed herein can provide a steady and uninterrupted supply ofelectricity to users for more than one week of time.

The Earth receives energy from the Sun that is more than 10,000 timesthe energy that all humans on the Earth consume. Wind energy is aderivative of the energy from the Sun. Yet, energy-hungry societiesdepend mostly on energy from burning fossil fuels. There is stronginternational pressure to reduce the consumption of fossil fuels and toswitch to renewable energy sources such as solar or wind power.

The cost of renewable energy is now roughly equal to or lower than thecost of energy generated by fossil fuels. However, a serious problem ofrenewable energy is that electricity generated by solar panels and windturbines cannot be stored economically for long period of time. Theserious problem of renewable energy boils down to the problem oflong-term economic methods of storing large amounts of the energy fromthe Sun. It is the problem of long-term economic storage of the energyfrom the Sun.

The capacity utilization factor (CUF) of renewable energy is low,averaging only about 17% to 20%. This is mainly due to day-and-nightcycles and frequently changing weather. A typical solar power planthaving a nominal capacity of 1 megawatt (1 MW) can generate about 990kilowatts (99% of 1 MW) of electricity at a peak time (e.g., around noonon a clear sunny day for solar panels). However, a few hours around noonare not the best time for maximum user consumption of the electricitygenerated by solar power plants. Sometimes, renewable energy is notavailable for many days when foul weather or windless conditionscontinue that long.

In the YKESS described herein, the strengths of FESS and MSESS aresynergistically enhanced and weaknesses of FESS and MSESS are eliminatedor minimized by novel integrations of FESS and MSESS. When appliedwidely in large scales, the YKESS described herein will eliminate ordrastically reduce the dependence of energy-hungry societies onenvironmentally destructive and unhealthy burning of fossil fuels.

The YKESS described herein include novel integrations of two existingwell-proven energy storage systems, namely, Flywheel Energy StorageSystems (FESS) and Molten Salt Energy Storage Systems (MSESS). The twotechnologies of FESS and MSESS are entirely different from each other.Designers and developers of FESS and MSESS have never worked together toachieve synergistic integration of the two energy storage systems.Therefore, no one in these two areas of energy storage systemstechnologies has uncovered incredible benefits of a novel andcounter-intuitive integration of FESS and MSESS as described in thisdisclosure. FESS and MSESS have characteristic strengths and weaknessesas two separate stand-alone storage systems of renewable energy.

One of the inventive concepts of the YKESS described in this disclosurecan be succinctly described as follows. The YKESS described hereinintegrates one or more low-cost FESS which stores only a relativelysmall amount of energy and a MSESS that stores a large amount of energyas low-cost heat energy of molten salt in such a way that the MSESSbecomes a large reservoir of energy and supplier of the energy to theFESS, when needed, and the FESS becomes a fast-responding provider of alarge amount of electric power to users and power grids almostinstantaneously in response to unpredictable and widely fluctuatingneeds of electricity during many days of foul weather for solar panelsor windless weather for wind turbines. Strengths and weaknesses of FESSand MSESS as two separate stand-alone systems, and novel integrations ofFESS and MSESS are described in the following sections.

Strengths and Weaknesses of Flywheel Energy Storage Systems (FESS)

A FESS can quickly generate and send electricity almost instantaneouslyto users when normal electric power is interrupted. The efficiency ofmodern FESS is high (e.g., in the range of 88% to 98%). A moderncommercially available FESS is highly reliable and durable. It isvirtually maintenance free for many years. Its service life is more than20 years. However, the energy storage capacity of a FESS is ratherlimited. The capacity of energy storage of FESS increases as the squareof the angular speed of its flywheel. For example, if the angular speedof a flywheel increases twice, the corresponding energy of the flywheelincreases four times. Therefore, a designer of FESS tries to maximizeangular speed of flywheel of a FESS. As of now, typical angular speedsof commercially viable FESSs are in the range of 5,000 rpm to 18,000 rpm(revolution per minute). When the angular speed of a flywheel of a FESSbecomes significantly greater than 18,000 rpm, the cost of making theFESS and operation cost of the FESS becomes prohibitively expensive dueto many difficult technical problems associated with a flywheel thatrotates at such a high angular speed. A commercially available FESS thatruns at an angular speed of 10,000 rpm can store only 32 kWh(kilowatt-hour). FESS is too expensive to store enough energy forlong-term (more than one day) continuous delivery of a reasonable amountof electric power to users.

A fast-rotating flywheel is dynamically unstable. A fast-rotatingflywheel is subject to gyroscopic precession and wobbling due to therotation of the Earth and other minor causes. The gyroscopic precessionand wobbling of the fast-rotating flywheel must be vigorously controlled(or suppressed) since the flywheel rotates at such a high angular speedin a tightly confined space of a vacuum chamber. Any failure of controlof the dynamics of the fast-rotating flywheel will result in acatastrophic accident or destruction. A fast-rotating flywheel becomes‘stiff’ in the sense that a change of direction of the flywheel's axisof rotation requires a stronger torque when the angular speed increases.Varieties of permanent magnets and electromagnets (magnets powered byelectricity) and sophisticated electronic control systems are used tocontrol (or regulate) gyroscopic precession and wobbling of thefast-rotation flywheel in a FESS. These electromagnetic control systemsconsume a lot of energy.

The total energy that can be stored by a FESS is proportional to thesquare of its flywheel's angular speed. However, increasing the angularspeed beyond about 20,000 rpm becomes extremely technically difficultand extremely expensive to operate. This is a fundamental limit of FESStechnology. There are R&D teams who try to make a FESS that runs at100,000 rpm and even higher. However, a FESS that runs at such a highangular speed is not technically and economically feasible forcommercial ESS applications in foreseeable future. On the other hand, acommercially viable FESS that runs at a technologically feasible angularspeed in the range of 5,000 rpm to 18,000 rpm has a rather limited totalenergy storage capacity.

One strength of FESS is that it can release its kinetic energy intolarge amounts of electric energy in a very short time to users (e.g.,within about one second) when a fast release of a large amount ofelectricity is needed by users. A FESS that operates at a modest angularspeed in the range of 5,000 rpm to 10,000 rpm is highly durable andalmost maintenance free. Its service life is over 20 years.

Although the energy release time of a FESS is excellent, the totalamount of energy a given FESS can deliver is quite limited. Forinstance, a commercially available FESS stores about 32 kWh and canrelease that energy at a rate of 8 kW. The total time that the FESS cansupply energy is, therefore, only about 4 hours. For a multi-megawattgrid application, such a power boost is excellent for smoothing outpower supply. However, the response to an electricity brownout (say, 10%drop in power in some regions) requires many units of FESS that areconnected to a large MSESS, as described herein.

Strengths and Weaknesses of Molten Salt Energy Storage Systems (MSESS)

MSESS is a well-proven technology for large scale and low-cost storageof renewable energy. Salt is abundantly available, and its price is low.Its heat capacity is large for the storage of a large amount of energyper unit weight. It is not harmful to humans or to fauna or flora ofecosystems.

A molten salt heat reservoir has a high storage efficiency (e.g., above90%). However, the efficiency of energy transformation from the heatenergy of molten salt to electricity is lower than 50%. On the otherhand, the cost of energy storage is about $30/kWh. In comparison, thecost of energy storage in a FESS is about $300/kWh and is trendingdownward.

The cost of Lithium-ion batteries has decreased dramatically, to about$137/kWh (about 5 times the cost of MSESS), but there are other seriousissues. Lithium-ion batteries are plagued with high temperaturedependence that can limit their practicality in cold climates and causethem to overheat in warm weather; they have a limited lifetime that canbe substantially less than 10 years; their use does not scale well inactual plant installations; and lithium-ion batteries depend on rarematerials, the mining and disposal of which causes major ecologicaldamage. The fact that lithium-ion batteries may need to be totallyreplaced several times in the lifetime of a FESS, or of a MSESS, isactually a multiple capital cost that may make long-term dependence onlithium-ion batteries for ESS unsuitable. In addition to repeatedcapital cost, there is inordinate, additional ecological damage.Accordingly, MSESS clearly shows advantages compared to lithium-ionbatteries for long term energy storage.

A major weakness of MSESS is that its response time is slow. Theresponse time of a MSESS is defined as a characteristic time of a MSESSto transit from its idle state to its operational state. In the idlestate, the MSESS does not generate electricity. In the operationalstate, the MSESS generates maximum electricity. A MSESS cannot quicklybe jumpstarted to be in its operational state from its idle state byitself. External power is needed to quickly jumpstart the compressor andpump system of an idle MSESS. The jumpstart of a MSESS involves fluidcompression, movement of a working fluid (e.g., steam or air) to a heatexchanger, and the slow process of heat exchange between the workingfluid and molten salt. For these reasons, the response time of a MSESSis in the range of 5 minutes to 20 minutes. The response time of verylarge MSESS is in range of a few hours. In addition, the efficiency ofMSESS decreases from is its normal efficiency when its generation ofelectricity changes in time. Therefore, a stand-alone MSESS cannotdeliver electricity fast enough to users when fast delivery of a largeamount of electricity is needed by users. This is a major weakness ofMSESS.

There are variations of MSESS systems (e.g., CAES—Compressed Air EnergyStorage) that improve the start-up times by using compressed air insteadof water, but they have their own major problems. For example, togenerate electricity by using only air instead of water one needs to usehuge volumes of air that need to be stored in huge underground cavernsunder extreme pressure. There are relatively few geographical areas ableto accommodate such installations. In addition, one loses the hugeexpansion associated with the vaporization of water into steam and thelatent heat effects of the huge heat of vaporization effects of water.To maintain short response times, such units must be operated in asteady spinning state that uses energy 100% of the time just to beready. That drastically reduces the overall efficiency of CAES.

The molten salt of a large MSESS can include thousands of tons of saltand occupies a large volume of space. Its ‘engine’ part, namely,compressor, pumps, turbine, and generator of electricity requirefrequent repairs and regular maintenance services. Dry air is thepreferred working fluid for the MSESS, although water-steam would be abetter working fluid for very large scale MSESS. The response time ofMSESS with dry air is significantly shorter than the response time ofheavy-duty large scale MSESS with water-steam.

Synergistic Integrations of FESS and MSESS (YKESS)

The YKESS described herein are synergistic, hybrid integrations of FESSand MSESS that dramatically enhance the individual strengths of FESS andMSESS, and eliminate or minimize the individual weaknesses of FESS andMSESS. The YKESS inventions described herein are new types ofeconomically-viable energy storage systems for the purpose of long-termenergy storage (e.g., storage of large amounts of energy from renewableenergy sources). The YKESS provide almost instant and non-interruptingdelivery of electricity to users for many days or many weeks whenrenewable energy cannot be harvested from the sun and/or wind due today-and-night cycles and/or many days or weeks of unfavorable weatherconditions.

The FESS, as a part of the YKESS, functions to provide instantaneousdelivery of large amounts of electricity to users. The MSESS, as a partof the YKESS, functions to provide low-cost storage of large amounts ofrenewable energy in the form of heat (of molten salt), to convert theheat into electricity, and to send the electricity to the FESS whenenergy stored in the FESS becomes depleted below a certain lowerthreshold level, and/or to send electricity from the MSESS to the users.

Since the energy of the YKESS is mostly stored in the molten salt of theMSESS, the FESS of the YKESS only needs to store a small amount ofenergy. For instance, if a stand-alone FESS can store 32 kWh and caninstantly deliver 8 kW to users for 4 hours, a counterpart FESS in theYKESS may store a small fraction of 32 kWh and can still deliver 8 kW ofimmediately available power for much longer than 4 hours, since itsenergy will be frequently or continuously replenished by electricitygenerated by the MSESS. Once the MSESS system is fully up to speed andis operating at 100% of its power capacity, the majority of the powerfrom the MSESS can bypass the FESS and supply the users directly. TheFESS may then be used to smooth the power supply to eliminatefluctuations in power for which the MSESS is not suitable because of itsdwell times.

Using the YKESS, brownouts may be eliminated for large data centers,large manufacturing plants, large hospitals, and the like. For example,if the YKESS adopts the same commercial FESS having a storage capacityof 32 kWh, it can deliver a power of 32 kW for 1 hour, or 64 kWh for 30minutes. Therefore, the YKESS can deliver a large amount of rapidlychanging electric power to its users for long period of time (e.g., aslong as 30% of heat energy stored in molten salt of MSESS is notcompletely depleted). This means that YKESS can meet the needs for aninstant large electric power of large hospitals, large data centers,etc., in the case of abrupt interruption of normal electric powers tosuch institutions. At present, most of such institutions usediesel-burning electric generators as their fast backup of electricity.YKESS can replace these air-polluting diesel generators in case majorportions of energy of these organizations are renewable energy already.

This feature of the YKESS has far-reaching positive impacts in FESSindustry. When the energy of the FESS is frequently or continuouslyreplenished by its accompanying MSESS that can carry much a largeramount of energy in its molten salt, the amount of energy the FESS maystore is only a small amount of energy while it can supply electricityto users for long time as long as the energy of the MSESS is notcompletely depleted.

The cost of energy storage in a large stand-alone FESS is much moreexpensive than cost of energy storage in a MSESS. That is a majorweakness of FESS. A strength of FESS is that it can reliably deliverlarge of amount electric power to users almost instantaneously inresponse to abrupt changes of electricity its users' needs. The low costof energy storage of the MSESS is its major strength. The YKESSdescribed herein are a synergistic integration of FESS and MSESS formaximum exploitation of the strengths of FESS and the strengths ofMSESS. The bottom line of the YKESS described herein is that one can usemuch smaller and much cheaper (in price, not in its quality) FESS toprovide instant and time-varying supply of large amounts of electricpower to users for very long times.

The YKESS described herein power the compressor and pump of the MSESS(e.g., to compress air and push compressed air into heat exchangers thatare immersed in the molten salt) by electricity generated by the FESS.Electric power that is generated by the MSESS and sent to the FESSvaries because the demand of electricity from users is time varying. Forthis reason, the over-all efficiency of YKESS as an integrated system ofthe FESS and the MSESS is better when the compressor and pump of theMSESS is powered by electricity from the FESS to eliminate the dwell andsynchronization times associated with MSESS alone.

The benefits of the YKESS described herein can be shown in the followingactual example of the synergistic effects of combining FESS and MSESS(with computations pertaining to a realistic case) as follows.

An example commercially available FESS has the following specs: Maximumenergy storage is 32 kWh, angular speed of its flywheel is 10,000 rpm,its weight is 5,000 kg (5 tons) and its housing dimension 132 cm×137 cm(height×width). The 5-ton FESS can deliver 8 kW of electric power tousers for 4 hours.

In the YKESS described herein, a FESS that is far smaller (e.g., farless weight than 5 ton) can deliver the same 8 kW to users many timeslonger than 4 hours. That is accomplished by the YKESS as follows inparagraphs (i) to (v):

(i) A MSESS has its characteristic transition time. The transition timeof a MSESS is defined as the time the MSESS takes to transition from itsidle state to its full operational state. It is typically up to 30minutes for a MSESS that generates electricity with a turbine that isdriven with hot and high-pressure air. For concreteness, it is assumedthat the transition time of the MSESS adopted in this example is 10minutes, and it can generate 20 kW to be sent to users and/or the FESSwhen the MSESS is fully operational.

(ii) The transition time of the MSESS and its maximum electric poweroutput determines the optimum (or recommended) energy storage capacityof the FESS in the YKESS described herein. The optimum (or recommended)energy storage capability of the FESS should be slightly greater thanthe electric energy that the MSESS can generate at its full electricpower during a time span of its transition time. Therefore, in thiscase, the electric power output of the MSESS is approximately 3.34 kWhduring a time span of 10 minutes (equal to its transition time of 10minutes).

(iii) Therefore, the optimal (or recommended) energy storage of the FESSin the example YKESS is just 4 kWh (which is slightly larger than 3.34kWh). The energy storage capacity of the FESS is much less than theenergy storage of 32 kWh of the ‘5-ton’ commercial FESS. If the MSESScan store 1000 kWh of energy in its hot molten salt, and the efficiencyof the MSESS is 30%, 300 kWh of the energy stored in the MSESS can besupplied to the FESS for 37.5 hours at a rate of 8 kW.

(iv) A stand-alone FESS that can send 8 kW for 37.5 hours continuouslyto users must have an energy storage capacity of 300 kWh. The 5-toncommercial FESS has energy storage capacity of just 32 kWh. Therefore,one would need more than 9 units of the 5-ton FESS (9.375 units exactly)with 32 kWh of storage for continuous delivery of 8 kW for 37.5 hours.The 9.375 units of 5-ton FESS will weigh 47 tons. From thesecomputations, it is clear that non-stop delivery of 8 kW to users withmultiple units of FESS is simply economically not feasible. This is whymany FESS are used for short-time delivery of large amount of electricpower to users.

(v) There is another drastic difference between the commercial FESS andsynergistic effects of the FESS and the MSESS in YKESS described herein.Since the FESS of YKESS may store just about 4 kWh of energy, it wouldweigh about 8 times less (32 kWh divided by 4 kWh) and its size will besignificantly smaller than the commercial stand-alone FESS. The amountof energy the flywheel can store is linearly proportional to its weight.This means that the FESS with 4 kWh of capacity used in this inventionwould weigh only about 625 kg (5000 kg divided by 8), and its sizeshould be significantly smaller than the 5-ton FESS. Obviously, theprice of the FESS of YKESS would be significantly lower than price ofthe example 5-ton commercial FESS.

As shown in the above series of computations, the novel integration ofFESS and MSESS brings in great benefits to the users of YKESS. Thepositive synergistic effects of the integration of FESS and MSES is notobvious at all. One must look at the two totally different energystorage systems critically and must make careful analysis of thesynergistic effects as described herein. This is why no one has everconceived of the integration of FESS and MSESS so far (to the bestknowledge of the inventors).

There is another important benefit of the synergistic integration ofFESS and MSESS. A single large MSESS can supply its electricity tomultiple units of FESS to many users. A large MSESS with a huge energystorage capacity is very slow when it comes to changing its electricoutput. In other words, its transition time is slow. It is in the rangeof 30 minutes to 3 hours. If the MSESS (solely) supplies its electricitydirectly to its users, it cannot change its output (power sent to theusers) fast enough in case power needs of its users change abruptly andunpredictably. The synergistic integration of FESS and MSESS of YKESScan solve this problem nicely as follows. The MSESS with its largeenergy storage capacity generates electricity and sends it to users andto many separately operating units of FESS. Each of the FESS will have asmall energy storage capacity. When multiple units of FESS receiveelectricity from a single MSESS, each of the FESS may have its ownenergy storage capacity that can be enough for the FESS to send 10 kW or20 kW to its users for about 10 minutes or so. Since the large MSESSsupplies its electricity to many units of FESS, the MSESS will runcontinuously. Therefore, each of the multiple units of FESS can ‘draw’electricity from the MSESS to replenish depleting energy levels forcontinuous delivery of electricity to users. Very importantly, each ofthe FESS supported by the large MSESS advantageously has small energystorage capacity (e.g., for the reasons described above).

The raw materials used to manufacture the YKESS described herein areenvironmentally friendly and abundant materials. Most materials thatwill be used for manufacturing of the YKESS described herein will benon-toxic to humans and to flora and fauna around its site. The servicelife of the FESS will be more than 20 years without any maintenance andrepairs. The service life of the MSESS will be well over 20 yearsprovided that turbine, compressor, pumps, and other electrical andmechanical components of the MSESS are properly maintained and quicklyrepaired when needed.

The MSESS in the YKESS described herein is essentially a large energyreservoir that supplies its energy to one or more FESS that can deliverelectric power fast and for a long time. However, the MSESS is not theonly option for its role in the YKESS. Any system capable of storing alarge amount of energy as heat energy (e.g., hot sands, hot gravels, orhot oil), and mechanical energy of compressed air can be adopted toYKESS as a large reservoir of energy to supply the FESS. However, MSESSis a preferred energy reservoir of YKESS since it is cheap forconstruction, it occupies relatively small and compact space, and it canbe installed at almost at any place (e.g., on a flat roof of a largebuilding, or underground in the backyard of a house).

FIG. 1 is a schematic diagram of an example YKESS 100 in accordance withsome embodiments. Arrowheads on single solid lines indicate the flow ofelectric power from one unit to another of the YKESS 100. Arrowheads ondouble solid lines indicate the flow of a fluid (e.g., air, steam, etc.)as a working fluid of the YKESS 100.

The YKESS 100 is used to store energy that is produced by one or moreenergy generation sources. For example, in FIG. 1, the YKESS 100 is usedin conjunction with a renewable energy source 200 (which can be solarpanels 210 and/or wind turbines 220, for example).

The YKESS 100 includes one or more FESS 110 (a single FESS 110 isdepicted here, but multiple FESS 110 can be included in someembodiments) and a MSESS 120. The FESS 110, as a part of the YKESS 100,functions to provide instantaneous delivery large amounts of electricityto the users 300. The MSESS 120, as a part of the YKESS 100, functionsto provide low-cost storage of large amounts of renewable energy in theform of heat (of molten salt), to convert the heat into electricity, andto send the electricity to users 300 and/or to the FESS 110 (e.g., whenenergy stored in the FESS 110 depletes below a certain level). Once theMSESS 120 is operating at or near 100% capacity, the MSESS 120 cansupply electricity directly to end users 300 to cover the bulk of thedemand from the users 300, while the FESS 110 can be used to providejust enough electricity to meet power demand surges/wrinkles and toensure synchronization. The split between providing energy from theMSESS 120 to the users 300 and to keep the FESS 110 fully chargedrequires a control system that is not depicted in FIG. 1.

As depicted, at least a first portion of the energy generated by therenewable energy source 200 can be sent to users 300 without poweringthe one or more FESS 110. Alternatively, or additionally, at least asecond portion of the energy generated from the renewable energy source200 can be sent to and stored in the FESS 110 and/or the MSESS 120.

Energy received by the FESS 110 (from the renewable energy source 200and/or the MSESS 120) is used to drive one or more flywheels that areused to generate electricity to be sent to users 300, and/or that areused to power a compressor or pump 136 of the MSESS 120. Energy receivedby the MSESS 120 (from the renewable energy source 200) is used to powera heater 220 that heats salt 122 (e.g., molten salt) within a container121 of the MSESS 120. In these ways, excess electrical power from therenewable energy source 200 can be stored in the FESS 110 and/or theMSESS 120.

In the YKESS 100 described herein, the FESS 110 can provide electricityto users 300 on a fast basis in response to interruptions of power fromthe renewable energy source 200. The FESS 110 can generate electricityquickly. In some cases, the response time of the FESS 110 is typicallyless than one second with high degree of efficiency. The FESS 110 candrive one or more generators to create electricity that is sent to users300 when power from the renewable energy source 200 is interpreted ordiminished by day-and-night cycles or by unpredictable weather.

In the YKESS 100 described herein, the MSESS 120 works as an economicaland high-capacity energy storage system that outputs electricity topower the FESS 110 and/or to be delivered directly to users 300. Moltensalt 122 in MSESS 120 can store a large amount of energy at a low cost.

Examples of how the YKESS 100 Works:

Assuming the FESS 110 has energy storage capacity available (i.e., theFESS 110 is/are not fully “charged”), electrical power from therenewable energy source 200 is sent to the FESS 110 when there is anexcessive leftover amount of electricity after the electricity is sentto meet the demand of the users 300 at any given moment of a day. Thepower sent to the FESS 110 drives the high-speed rotations of the one ormore flywheels of the FESS 110. Accordingly, electricity from therenewable energy source 200 is stored by the FESS 110.

Additionally, some of the excessive leftover amount of electricity fromthe renewable energy source 200 is sent directly to the heater 220 ofthe MSESS 120 to heat its molten salt 122 (e.g., whenever it isnecessary to prevent the temperature of the molten salt 122 from coolingdown below a predetermined or desired temperature of the molten salt122). In some embodiments, in addition to the salt 122 in the container121, there is an inert gas 124 such as nitrogen or carbon dioxide in thecontainer 121

The energy storage capacity of the FESS 110 is usually quite a lotsmaller than the energy storage capacity of the MSESS 120. In part, thistends to be the case since the cost of energy storage in the FESS 110 issignificantly higher than the cost of energy storage in the MSESS 120.Therefore, the energy storage capacity of the MSESS 120 is much largerthan the energy storage capacity of the FESS 110 in the YKESS 100.

When the energy-storing capacity of the FESS 110 is fully loaded, theYKESS 100 can automatically divert the remaining leftover amount ofelectricity from the renewable energy source 200 to the MSESS 120 (whichcan store a lot more energy than the FESS 110). In addition, the MSESS120 receives electricity from the renewable energy source 200 when thetemperature of its molten salt 122 goes below a certain pre-determinedlower threshold value.

The MSESS 120 stores energy as follows. The MSESS 120 receiveselectricity directly from the renewable energy source 200. The MSESS 120converts the electricity into heat energy in its molten salt 122 usingthe Ohmic heater 220 that is in thermal contact the salt 122 of theMSESS 120. In this way, the temperature of the molten salt 122 ismaintained at or above a desired value. The molten salt 122 is containedin the container 121 (e.g., tank, vessel, etc.). The container 121 iswell insulated against heat loss. Very good heat insulation can beachieved cost effectively by wrapping up the container 121 with a lowcost and lightweight insulating materials such as fiberglass,polyurethane foam, or more expensive materials such as aerogel andPyrogel™.

In the YKESS 100 described herein, excellent heat insulation of thecontainer 121 is beneficial since the MSESS 120 is used to store largeamounts of heat energy for several weeks, or even for a few months. Inone example, a targeted degree of thermal insulation of the container121 is used to achieve a cooling rate of 10 C to 30 C (centigrade) orless per week, when temperature of the molten salt is in the range of500 C to 900 C. The energy retention capabilities of the MSESS 120 willdepend on factors such as the actual volume and surface area of thecontainer 121 of molten salt 122, and the degree of heat insulation ofinsulating materials that wrap around the container 121. A thermallywell-insulated MSESS 120 having a very large energy storage capacity asa source of electrical energy generation (for long-term delivery ofelectricity to users 300 in case of longtime interruptions ofelectricity generation from the renewable energy source 200) to powerthe FESS 110 is a novel feature of the YKESS 100 described herein.

The depicted MSESS 120 converts its heat energy into electricity asfollows. As shown in FIG. 1, the MSESS 120 has its owncompressed-air-driven-turbine (CADT) system 130. Accordingly, the CADTsystem 130 can be considered as a subsystem of the MSESS 120. Thedepicted CADT system 130 includes a turbine 132, a generator 133, an airstorage tank 134, a compressor pump 136, and a heat exchanger 138.

A working fluid (e.g., air or steam; air is used in this example) in thepiping of the CADT system 130 is compressed by the compressor pump 136(which is powered by the FESS 110) to push the air into the heatexchanger 138 (e.g., one or more heat exchanging pipes or tubing) thatis in contact with (e.g., immersed) in the molten salt 122 of the MSESS120. As the air goes through the heat exchanger 138 (usually made ofstainless steel), the air is heated by heat stored in the molten salt122 to a high temperature and raised to a high pressure. Thissuperheated air (air with high temperature and high pressure) is thensent to the turbine 132 of the CADT system 130. The turbine 132 drivesthe generator 133 to create electricity that is transmitted to the FESS110 (to drive rotations of one or more flywheels of the FESS 110). Asthe hot air drives the turbine 132, the air expands and cools. If theair has a negligible amount of moisture, or has no moisture at all, itcan be considered as an ideal gas in the temperature in range of 500 Cto 900 C. If the air contains a non-negligible amount of moisture, theair may not be considered as an ideal gas, and efficiency of the CADTsystem 130 will be lower than the efficiency of the CADT system 130 withdry air. Therefore, for the YKESS 100 described herein, it isadvantageous for the air in the CADT system 130 to be dry air, or tohave a low level of moisture.

In some embodiments, after the air in the CADT system 130 is dischargedfrom the turbine 132 the air is passed through an array of undergroundpipes to cool the air before its next use in the generator cycle. It isimportant that the air be dry to prevent condensation during thiscooling step.

Since the air in the CADT system 130 can be considered an ideal gas asdescribed above, the total energy contained in a unit mass of the air isproportional to its temperature (absolute temperature). Let T1 be thetemperature of the air before it enters the heat exchanger 138 in theMSESS 120. Let T2 be the temperature of the air as it expands and drivesthe turbine 132. Clearly, a higher value of T2 and a lower value of T1will result in higher efficiency of the CADT system 130. Accordingly,the rate of energy delivered by the air to the turbine 132 isproportional to (T2−T1) multiplied by flow rate of the air (e.g., themass of the air that goes to the turbine blades per unit time), sincethe viscosity loss of the air is negligible as an ideal gas. A modernair compressor (such as the compressor pump 136) is highly efficient(e.g., 95% or higher). If we were to assume the power to drive thecompressor pump 136 comes from the generator 133 of the CADT system 130(contrary to the design of the YKESS 100), the total efficiency of theCADT system 130 would be about 70%.

However, in the YKESS 100 described herein, the energy used to run theair compressor pump 136 of the CADT system 130 will be typicallysupplied by electricity (or by direct mechanical power transmission)from the FESS 110, not by the electricity generated from the CADT system130 in conjunction with the MSESS 120. This is very beneficial becausethe efficiency of the MSESS 120 is about 70%, while efficiency of theFESS 110 is about 95% (or even higher than 95%). For example, supposethe MSESS 120 must use 100 watts of power to run the compressor pump 136of the CADT system 130. In that case, the MSESS 120 in conjunction withthe turbine 132 and the generator 133 must produce about 143 watts ofpower to deliver the 100 watts to run the compressor pump 136. This isbecause the efficiency of the MSESS 120 is about 70%. However, the FESS110 can deliver the same power of 100 watts to the compressor pump 136by consuming about 105 watts (to produce the 100 watts). There is adifference of about 38 watts (143 watts minus 105 watts) between theMSESS 120 and the FESS 110 when each of the two forms of ESS delivers100 watts to the compressor pump 136 independently. In other words,powering the air compressor pump 136 of the CADT system 130 using energyfrom the FESS 110 is much more efficient than by using energy from theMSESS 120.

Using the YKESS 100 described herein, the MSESS 120 does not need topower the air compressor pump 136 of the CADT system 130 (although itcan in some cases). Instead, the MSESS 120 gets the compressed airwithout spending its own energy, since it is the FESS 110 that providesthe energy to compress the air. This means that MSESS 120 can generateelectricity (via the turbine 132 and the generator 133) withsignificantly higher efficiency when the air compressor pump 136 ispowered by the FESS 110. This is a novel unique feature of the YKESS 100described herein.

After leaving the turbine 132, the now colder air enters the storagetank 134 before it goes back to the air compressor pump 136 of the CADTsystem 130. In some embodiments, the temperature (T3) of the air in thestorage tank 134 will be essentially at ambient temperature. When theair is compressed by the air compressor pump 136 of the CADT system 130(before it enters the heat exchanger 138 in the MSESS 120), the air isthereby heated to the new temperature of T1 (as described above).However, the work to raise the air temperature from T3 to T1 is done byFESS 110, not by the MSESS 120. Therefore, the theoretical efficiency ofthe CADT system 130 will be (T2−T3)/T2 rather than (T2−T1)/T2. Since T3is lower than T1, the theoretical efficiency of the MSESS 120 withoutspending its own energy to run the air compressor pump 136 issignificantly higher than its theoretical efficiency with spending itsown energy to run the air compressor pump 136. Since the dry air used inCADT system 130 can be considered as an ideal gas, the actual efficiencyof the MSESS 120 will be nearly equal to its theoretical value. In thisway, the MSESS 120 can generate its electricity with significantlyhigher efficiency than a conventional stand-alone MSES can generate.

In some cases, such as when an interruption of electricity generation bythe renewable energy source 200 continues for many days and the energystorage of the FESS 110 becomes very low, the air compressor pump 136will automatically switch from being powered by the FESS 110 to beingpowered by electricity generated by the MSESS 120 and the CADT system130 (from the generator 133). This will usually be a rare event in thereal world. In conventional MSESS, the energy for its air compressorpump is often supplied by an outside source such as an independentelectric generator powered by burning fossil fuel. This runs contrary tothe purpose of environmentally friendly renewable energy from thesunlight or wind. This is another benefit of the YKESS 100 describedherein.

The generation of electricity by the renewable energy source 200 tendsto fluctuate almost constantly due changing weather and day-and-nightcycles. An interruption of the electricity generated by the renewableenergy source 200 may last for minutes, a few hours, a few days, or afew weeks. In this embodiment of the YKESS 100, the FESS 110 alonestores enough energy for a few hours of supply of electricity to theuser 300, and the MSESS 120 is used to store enough energy to supplyelectricity to drive the flywheel(s) of the FESS 110, and/or to supplythe users 300, for several days or even for a few weeks. Accordingly,the YKESS 100 can provide electricity to users 300 from the FESS 110(when powered by the MSESS 120) and/or from the MSESS 120 for severaldays or even for a few weeks when there are such interruptions in thegeneration of electricity by the renewable energy source 200.

It should be noted that the YKESS 100 described herein is not limited byexact values of efficiency of the FESS 110 or the MSESS 120 (e.g., asdescribed above) since exact values of efficiency of the FESS 110 andthe MSESS 120 will depend on details of designs and work conditions ofthe FESS 110 and the MSESS 120, and/or because it is almost impossibleto measure exact values of efficiency of the FESS 110 and the MSESS 120.However, the efficiency numbers mentioned in this document arereasonably estimated numbers. The novel features of the YKESS 100described herein are independent of exact values of efficiency of theFESS 110 and the MSESS 120, and material properties mentioned in thisdocument.

The description of the YKESS 100 described herein is provided in thecontext of a solar or wind power plant. However, the energy from otherenergy sources can be stored in the same type or similar types ofintegrated YKESS 100, as described herein. In the YKESS 100 describedherein, the MSESS 120 may be replaced with large-scale compressed airstorage (CAES) if it is more cost effective for the same role of theMSESS 120.

FIG. 2 is a schematic diagram of another example embodiment of YKESS100′. In this version, ambient air is sucked into a filter anddehumidifier 135, compressed in the compressor or pump 136, heated inthe heat exchanger 138, and then discarded into open ambient air afterit passes the turbine 132 where it releases its energy to power thegenerator 133. Again, electricity output from the generator 133 is sentto the one or more FESS 110 to drive the rotations of the one or moreflywheels of the FESS 110. The one or more FESS 110, in turn, generateelectricity that is sent to the users 300 and generate power that isused to drive the compressor or pump 136. As in the case of YKESS 100,energy from the generator 133 can be fed to the FESS 110 (to increasethe kinetic energy of the FESS 110) and/or to the users 300.

FIG. 3 is a block diagram that illustrates that a large scale YKESS 100″can include a MSESS 120 that is operatively connected to a plurality ofFESSs 110 in the same manner as described elsewhere herein (e.g., whereelectricity generated by the MSESS 120 is used to power rotations of theflywheels of the FESSs 110, and power from the FESSs 110 drives a pumpor compressor of the MSESS 120). The multiple units of FESS 110 areconnected to provide electricity to end users 300 of electricity orpower grids 300. While not shown for simplicity sake, the electricitygenerated by the renewable energy source 200 can also be delivereddirectly to each FESS 110 of the plurality of FESSs 110 to powerrotations of the flywheels of the FESSs 110. Also, for simplicity, thefigure does not show the MSESS powering the users 300 directly.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of anyinvention or of what may be claimed, but rather as descriptions offeatures that may be specific to particular embodiments of particularinventions. Certain features that are described in this specification inthe context of separate embodiments can also be implemented incombination in a single embodiment. Conversely, various features thatare described in the context of a single embodiment can also beimplemented in multiple embodiments separately or in any suitablesub-combination. Moreover, although features may be described herein asacting in certain combinations and even initially claimed as such, oneor more features from a claimed combination can in some cases be excisedfrom the combination, and the claimed combination may be directed to asub-combination or variation of a sub-combination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various modules andcomponents in the embodiments described herein should not be understoodas requiring such separation in all embodiments, and it should beunderstood that the described components and systems can generally beintegrated together in a single product or packaged into multipleproducts.

Particular embodiments of the subject matter have been described. Otherembodiments are within the scope of the following claims. For example,the actions recited in the claims can be performed in a different orderand still achieve desirable results. As one example, the processesdepicted in the accompanying figures do not necessarily require theparticular order shown, or sequential order, to achieve desirableresults. In certain implementations, multitasking and parallelprocessing may be advantageous.

What is claimed is:
 1. An energy storage and delivery system comprising: one or more flywheel energy storage systems (FESS), the one or more FESS including or coupled to one or more generators configured to generate and deliver electricity to users; and a heat energy storage system (HESS) comprising: a container holding a heat storage medium therein; a heater in thermal contact with the heat storage medium such that the heat storage medium is heated in response to the heater receiving energy from an energy source; and a heat exchanger in thermal contact with the heat storage medium; a turbine fluidly coupled to an output of the heat exchanger; an electricity generator mechanically coupled to the turbine, wherein electricity output by the electricity generator is delivered to the one or more FESS to power flywheel rotations of the one or more FESS; and a compressor or pump fluidly coupled to an input of the heat exchanger, wherein the compressor or pump is at least partially powered by energy output by the one or more FESS.
 2. The system of claim 1, wherein the heater is configured to be electrically powered by electricity generated by solar panels or wind turbines.
 3. The system of claim 2, wherein the system further comprises the solar panels or the wind turbines.
 4. The system of claim 3, wherein the system is configured to send at least a first portion of the electricity generated by the solar panels or the wind turbines to users.
 5. The system of claim 4, wherein system is configured to send at least a second portion the electricity generated by the solar panels or the wind turbines to the one or more FESS to power flywheel rotations of the one or more FESS.
 6. The system of claim 1, wherein the heater is configured to transfer heat to the heat storage medium.
 7. The system of claim 1, wherein the heat exchanger is configured to transfer heat from the heat storage medium to a fluid passing through the heat exchanger as the fluid flows toward the turbine.
 8. The system of claim 1, further comprising a fluid storage tank positioned fluidly between an output of the turbine and an input of the compressor or pump.
 9. The system of claim 1, wherein the heat storage medium comprises salt, sand, gravel, or oil.
 10. The system of claim 1, wherein the system is configured to receive energy from a renewable energy source and to distribute the energy to: (i) the one or more FESS for storage of the energy delivered thereto, and (ii) the HESS system for storage of the energy delivered thereto, and wherein the HESS has an energy storage capacity that is at least two times more than the one or more FESS.
 11. The system of claim 1, wherein the HESS is also configured to deliver energy to power the compressor or pump.
 12. The system of claim 11, wherein the system includes a control system that is configured to automatically switch the energy to power the compressor or pump from being delivered by the one or more FESS to being delivered by the HESS in response to the energy contained by the one or more FESS reaching a lower limit.
 13. The system of claim 1, wherein the electricity output by the electricity generator is also delivered to the users.
 14. The system of claim 1, wherein the one or more FESS comprises at least four FESS.
 15. A method of delivering electricity to users, the method comprising: delivering a first amount of the energy generated by a renewable energy source to one or more flywheel energy storage systems (FESS); storing, by the one or more FESS, the first amount of the energy as kinetic energy; delivering a second amount of the energy generated by the renewable energy source to a heat energy storage system (HESS); storing, by the HESS, the second amount of the energy as heat energy; generating electricity using the heat energy stored by the HESS; and delivering, to the users, a first amount of the electricity that was generated using the heat energy stored by the HESS.
 16. The method of claim 15, wherein a second amount of the electricity that was generated using the heat energy stored by the HESS is delivered to the one or more FESS in response to a kinetic energy level of the one or more FESS being below a pre-determined lower threshold value.
 17. The method of claim 15, further comprising powering, by energy delivered from the one or more FESS, a pump or compressor of the HESS.
 18. The method of claim 15, further comprising, in response to a demand for electricity from the users being greater than what the renewable energy source is delivering, delivering additional electricity to the users, wherein the additional electricity is generated by the one or more FESS.
 19. The method of claim 15, wherein the first amount of the electricity that was generated using the heat energy stored by the HESS is delivered to the users in response to a demand for electricity from the users being greater than what the renewable energy source is delivering. 